Windows Register List fixes

make the rightclick work in the registerlist for all displayed registers.
The LO HI and PC registers couldn't be rightclicked previously.

Also, if you rightlick a valid row and then rightclick on the leftmost 16
pixels you'd get the popup menu for the old selection (even on invalid y
indices).

Make the "change register value"-dialog start with '0x' because otherwise
values starting with an alphabetical hexadecimal symbol will yield
be parsed as a symbol instead of a number and yield 'unknown symbol'
Merge pull request #4534 from Bigpet/WinRegisterlistFix

Windows Register List fixes